(1) Field of the Invention
The invention relates to the general field of Liquid Crystal Displays, more particularly to the angular distribution of the light emerging from such displays.
(2) Description of the Prior Art
Referring to FIG. 1a, the basic parts of a liquid crystal display are schematically illustrated in cross-section. A number of layers are involved, the outermost being a pair of light polarizers, entrance polarizer 1 and exit polarizer 2. In their most commonly used configuration, the polarizers are arranged so as to have their optic axes orthogonal to one another. That is, in the absence of anything else between them, light passing through the entrance polarizer would be blocked by the exit polarizer, and vice versa.
Immediately below the entrance polarizer is an upper transparent insulating substrate 3 (usually glass) and immediately above the exit polarizer is a similar lower substrate 4. Transparent conducting lines 6 and 7, usually comprising indium tin oxide (ITO) run orthogonal to one another and are located on the lower surface of 3 and the upper surface of 4, respectively. Lines 6 run perpendicular to the plane of FIG. 1a while lines 7 run parallel to it.
Sandwiched between, and confined there by means of suitable enclosing walls (not shown), is a layer of liquid crystal 5 such as, for example, twisted nematic (TN) or super twisted nematic (STN) liquid crystal molecules. The orientation of these molecules, relative to a given surface can be controlled by coating such a surface with a suitable orientation layer (18 and 19 in FIG. 1a) and rubbing said orientation layer in the desired direction just prior to bringing it into contact with the liquid crystals.
Thus, in FIG. 1a, the molecules closest to upper substrate 3 have been oriented so as to lie in the plane of the figure while the molecules closest to lower substrate 4 have been oriented to lie perpendicular to this plane. Molecules in between the two sets of pre-oriented molecules then arrange themselves so as to gradually change their orientations between these two extremes. Hence the term `twisted nematic` (TN) for such a configuration. A TN is optically active and will rotate the plane of any polarized light that traverses it.
Thus, polarized light that was formed and oriented as a result of passing through entrance polarizer 1 will be rotated though an angle of 90.degree. after traversing layer 5 and so will be correctly oriented to pass through exit polarizer 2. Such a device is therefore normally on (transmits light).
An important property of TN is that, in the presence of an electric field (typically about 1,000 volts/cm.), normal to the the molecules, said molecules will all orient themselves so as to point in the same direction and the liquid crystal layer will cease to be optically active. By using orthogonal lines as electrodes for said electric field it becomes possible to confine such a field to the intersection of any given pair of these lines. Such an intersection, where lines 6 and lines 7 overlap, then constitutes a single pixel of the display.
To view a display of the type illustrated in FIG. 1a, light may be applied from above polarizer 1 and then viewed from below polarizer 2 or a reflecting surface may be applied to the lower surface of polarizer 2 and the device viewed from above polarizer 1. In either case, the exiting light tends to be largely orthogonal to the surface of 1 and the brightness of the display falls off rapidly as the viewing angle increases. As an example of this see curve 9 in FIG. 1b which is taken from some prior art data of Kozaki (see below). Using an arbitrary contrast ratio scale, it is seen that the observed brightness of the display is at a maximum at normal, or close to normal, viewing angles, and falls off rapidly for viewing angles greater than about 15.degree., going to zero for a viewing angle of 30.degree. when the display is viewed in a downward direction and about 50.degree. (extrapolated) when the display is viewed in an upward direction. Note that this asymmetry in the viewing angle curves is a consequence of the fact that all TNs are oriented in the same direction at the two orientation surfaces, 18 and 19.
A number of approaches to improving the viewing angles of liquid crystal devices have been described in the patent literature. For example, Kozaki et al. (U.S. Pat. No. 5,124,824 Jun 1992), referred to above, teach that an improvement in viewing angle may be achieved by means of a retardation compensation layer, optionally used in conjunction with a phase difference cancellation film. This allows the absolute brightness of the display to be increased, including the portions that emerge at an oblique angle.
Castleberry (U.S. Pat. No. 5,107,356 Apr 1992) uses, in addition to polarizers, birefringent films that convert the linearly polarized entering light to elliptically polarized light, said process being reversed when light exits the display. This results in an improved contrast ratio between the on and off states in all directions, including the oblique.
Takano (U.S. Pat. No. 5,244,070 Sep 1993) modifies the composition of the liquid crystal material through the addition of a chiral dopant. This causes groups of molecules in the TN to exhibit, in roughly equal proportions, both right-handed and left-handed twists. As a result, in this device, the curve of FIG. 1b becomes symmetrical and the viewing angle distribution curve has the values normally associated only with upward viewing (in FIG. 1b) for both directions.
Note that Takano mentions the possibility of covering the entire panel with a single Fresnel lens as a possible way to improve the angular distribution of the emerging light but dismisses this as too expensive as well as disagreeable from the point of view of viewing comfort.